My system language is set to US English, but the spellchecker in Adium insists on adhering to UK English (e.g., it marks "color" as being incorrect and wants to substitute it with "colour"). I don't have this problem with any other apps.

Check Edit > Spelling > Show Spelling and Grammar and see if it is set to Automatic By Language or British English..You can also check System Preferences > Keyboard > Text to see what your system is set up for, but based on your post, it sounds like it's just an Adium setting..